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10 Derived classes [class.derived]
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1 A list of base classes can be specified in a class declaration using
the notation:
base-clause:
: base-specifier-list
base-specifier-list:
base-specifier
base-specifier-list , base-specifier
base-specifier:
::opt nested-name-specifieropt class-name
virtual access-specifieropt ::opt nested-name-specifieropt class-name
access-specifier virtualopt ::opt nested-name-specifieropt class-name
access-specifier:
private
protected
public
The class-name in a base-specifier shall denote a previously declared
class (_class_), which is called a direct base class for the class
being declared. A class B is a base class of a class D if it is a
direct base class of D or a direct base class of one of D's base
classes. A class is an indirect base class of another if it is a base
class but not a direct base class. A class is said to be (directly or
indirectly) derived from its (direct or indirect) base classes. For
the meaning of access-specifier see _class.access_. Unless redefined
in the derived class, members of a base class can be referred to in
expressions as if they were members of the derived class. The base
class members are said to be inherited by the derived class. The
scope resolution operator :: (_expr.prim_) can be used to refer to a
base member explicitly. This allows access to a name that has been
redefined in the derived class. A derived class can itself serve as a
base class subject to access control; see _class.access.base_. A
pointer to a derived class can be implicitly converted to a pointer to
an accessible unambiguous base class (_conv.ptr_). An lvalue of a
derived class type can be bound to a reference to an accessible unam
biguous base class (_dcl.init.ref_).
2 For example,
class Base {
public:
int a, b, c;
};
class Derived : public Base {
public:
int b;
};
class Derived2 : public Derived {
public:
int c;
};
3 Here, an object of class Derived2 will have a sub-object of class
Derived which in turn will have a sub-object of class Base. A derived
class and its base class sub-objects can be represented by a directed
acyclic graph (DAG) where an arrow means directly derived from. A DAG
of sub-objects is often referred to as a sub-object lattice. For
example,
Base
|
|
Derived
|
Derived2
Note that the arrows need not have a physical representation in memory
and the order in which the sub-objects appear in memory is unspeci
fied.
4 Initialization of objects representing base classes can be specified
in constructors; see _class.base.init_.
10.1 Multiple base classes [class.mi]
1 A class can be derived from any number of base classes. For example,
class A { /* ... */ };
class B { /* ... */ };
class C { /* ... */ };
class D : public A, public B, public C { /* ... */ };
The use of more than one direct base class is often called multiple
inheritance.
2 The order of derivation is not significant except possibly for ini
tialization by constructor (_class.base.init_), for cleanup
(_class.dtor_), and for storage layout (_expr.cast_, _class.mem_,
_class.access.spec_).
3 A class shall not be specified as a direct base class of a derived
class more than once but it can be an indirect base class more than
once.
class B { /* ... */ };
class D : public B, public B { /* ... */ }; // ill-formed
class L { public: int next; /* ... */ };
class A : public L { /* ... */ };
class B : public L { /* ... */ };
class C : public A, public B { void f(); /* ... */ }; // well-formed
For an object of class C, each distinct occurrence of a (non-virtual)
base class L in the class lattice of C corresponds one-to-one with a
distinct L subobject within the object of type C. Given the class C
defined above, an object of class C will have two sub-objects of class
L as shown below.
L L
| |
| |
A B
C
In such lattices, explicit qualification can be used to specify which
subobject is meant. For example, the body of function C::f could
refer to a member next of each l subobject:
void C::f() { A::next = B::next; } // well-formed
Without the A:: or B:: qualifiers, the definition of C::f above would
be ill-formed because of ambiguity.
4 The keyword virtual can be added to a base class specifier. A single
sub-object of the virtual base class is shared by every base class
that specified the base class to be virtual. For example,
class V { /* ... */ };
class A : virtual public V { /* ... */ };
class B : virtual public V { /* ... */ };
class C : public A, public B { /* ... */ };
Here class C has only one sub-object of class V, as shown below.
V
A B
C
5 A class can have both virtual and nonvirtual base classes of a given
type.
class B { /* ... */ };
class X : virtual public B { /* ... */ };
class Y : virtual public B { /* ... */ };
class Z : public B { /* ... */ };
class AA : public X, public Y, public Z { /* ... */ };
For an object of class AA, all virtual occurrences of base class B in
the class lattice of AA correspond to a single B subobject within the
object of type AA, and every other occurrence of a (non-virtual) base
class B in the class lattice of AA corresponds one-to-one with a dis
tinct B subject within the object of type AA. Given the class AA
defined above, class AA has two sub-objects of class B: Z's B and the
virtual B shared by X and Y, as shown below.
B B
|
|
X Y Z
AA
10.2 Member Name Lookup [class.member.lookup]
1 Member name lookup determines the meaning of a name ( id-expression)
in a class scope. Name lookup can result in an ambiguity, in which
case the program is ill-formed. For an id-expression, name lookup
begins in the class scope of this; for a qualified-id, name lookup
begins in the scope of the nested-name-specifier. Name lookup takes
place before access control (_class.access_).
2 The following steps define the result of name lookup in a class scope.
First, we consider every declaration for the name in the class and in
each of its base class sub-objects. A member name f in one sub-object
B hides a member name f in a sub-object A if A is a base class sub-
object of B. We eliminate from consideration any declarations that
are so hidden. If the resulting set of declarations are not all from
sub-objects of the same type, or the set has a nonstatic member and
includes members from distinct sub-objects, there is an ambiguity and
the program is ill-formed. Otherwise that set is the result of the
lookup.
3 For example,
class A {
public:
int a;
int (*b)();
int f();
int f(int);
int g();
};
class B {
int a;
int b();
public:
int f();
int g;
int h();
int h(int);
};
class C : public A, public B {};
void g(C* pc)
{
pc->a = 1; // error: ambiguous: A::a or B::a
pc->b(); // error: ambiguous: A::b or B::b
pc->f(); // error: ambiguous: A::f or B::f
pc->f(1); // error: ambiguous: A::f or B::f
pc->g(); // error: ambiguous: A::g or B::g
pc->g = 1; // error: ambiguous: A::g or B::g
pc->h(); // ok
pc->h(1); // ok
}
4 If the name of an overloaded function is unambiguously found, over
loading resolution also takes place before access control. Ambigui
ties can often be resolved by qualifying a name with its class name.
For example,
class A {
public:
int f();
};
class B {
public:
int f();
};
class C : public A, public B {
int f() { return A::f() + B::f(); }
};
5 The definition of ambiguity allows a nonstatic object to be found in
more than one sub-object. When virtual base classes are used, two
base classes can share a common sub-object. For example,
class V { public: int v; };
class A {
public:
int a;
static int s;
enum { e };
};
class B : public A, public virtual V {};
class C : public A, public virtual V {};
class D : public B, public C { };
void f(D* pd)
{
pd->v++; // ok: only one `v' (virtual)
pd->s++; // ok: only one `s' (static)
int i = pd->e; // ok: only one `e' (enumerator)
pd->a++; // error, ambiguous: two `a's in `D'
}
6 When virtual base classes are used, a hidden declaration can be
reached along a path through the sub-object lattice that does not pass
through the hiding declaration. This is not an ambiguity. The
identical use with nonvirtual base classes is an ambiguity; in that
case there is no unique instance of the name that hides all the oth
ers. For example,
class V { public: int f(); int x; };
class W { public: int g(); int y; };
class B : public virtual V, public W
{
public:
int f(); int x;
int g(); int y;
};
class C : public virtual V, public W { };
class D : public B, public C { void glorp(); };
W V W
B C
D
The names defined in V and the left hand instance of W are hidden by
those in B, but the names defined in the right hand instance of W are
not hidden at all.
void D::glorp()
{
x++; // ok: B::x hides V::x
f(); // ok: B::f() hides V::f()
y++; // error: B::y and C's W::y
g(); // error: B::g() and C's W::g()
}
7 An explicit or implicit conversion from a pointer to or an lvalue of a
derived class to a pointer or reference to one of its base classes
shall unambiguously refer to a unique object representing the base
class. For example,
class V { };
class A { };
class B : public A, public virtual V { };
class C : public A, public virtual V { };
class D : public B, public C { };
void g()
{
D d;
B* pb = &d;
A* pa = &d; // error, ambiguous: C's A or B's A ?
V* pv = &d; // fine: only one V sub-object
}
10.3 Virtual functions [class.virtual]
1 Virtual functions support dynamic binding and object-oriented program
ming. A class that declares or inherits a virtual function is called
a polymorphic class.
2 If a virtual member function vf is declared in a class Base and in a
class Derived, derived directly or indirectly from Base, a member
function vf with the same name and same parameter list as Base::vf is
declared, then Derived::vf is also virtual (whether or not it is so
declared) and it overrides1) Base::vf. For convenience we say that
any virtual function overrides itself. Then in any well-formed class,
for each virtual function declared in that class or any of its direct
or indirect base classes there is a unique final overrider that over
rides that function and every other overrider of that function.
3 A virtual member function does not have to be visible to be overrid
den, for example,
struct B {
virtual void f();
};
struct D : B {
void f(int);
};
struct D2 : D {
void f();
};
the function f(int) in class D hides the virtual function f() in its
base class B; D::f(int) is not a virtual function. However, f()
declared in class D2 has the same name and the same parameter list as
B::f(), and therefore is a virtual function that overrides the func
tion B::f() even though B::f() is not visible in class D2.
4 A program is ill-formed if the return type of any overriding function
differs from the return type of the overridden function unless the
return type of the latter is pointer or reference (possibly cv-
qualified) to a class B, and the return type of the former is pointer
or reference (respectively) to a class D such that B is an unambiguous
direct or indirect base class of D, accessible in the class of the
overriding function, and the cv-qualification in the return type of
the overriding function is less than or equal to the cv-qualification
in the return type of the overridden function. In that case when the
overriding function is called as the final overrider of the overridden
function, its result is converted to the type returned by the (stati
cally chosen) overridden function. See _expr.call_. For example,
_________________________
1) A function with the same name but a different parameter list (see
_over_) as a virtual function is not necessarily virtual and does not
override. The use of the virtual specifier in the declaration of an
overriding function is legal but redundant (has empty semantics). Ac
cess control (_class.access_) is not considered in determining over
riding.
class B {};
class D : private B { friend class Derived; };
struct Base {
virtual void vf1();
virtual void vf2();
virtual void vf3();
virtual B* vf4();
void f();
};
struct No_good : public Base {
D* vf4(); // error: B (base class of D) inaccessible
};
struct Derived : public Base {
void vf1(); // virtual and overrides Base::vf1()
void vf2(int); // not virtual, hides Base::vf2()
char vf3(); // error: invalid difference in return type only
D* vf4(); // okay: returns pointer to derived class
void f();
};
void g()
{
Derived d;
Base* bp = &d; // standard conversion:
// Derived* to Base*
bp->vf1(); // calls Derived::vf1()
bp->vf2(); // calls Base::vf2()
bp->f(); // calls Base::f() (not virtual)
B* p = bp->vf4(); // calls Derived::pf() and converts the
// result to B*
Derived* dp = &d;
D* q = dp->vf4(); // calls Derived::pf() and does not
// convert the result to B*
dp->vf2(); // ill-formed: argument mismatch
}
5 That is, the interpretation of the call of a virtual function depends
on the type of the object for which it is called (the dynamic type),
whereas the interpretation of a call of a nonvirtual member function
depends only on the type of the pointer or refe rence denoting that
object (the static type). See _expr.call_.
6 The virtual specifier implies membership, so a virtual function cannot
be a global (nonmember) (_dcl.fct.spec_) function. Nor can a virtual
function be a static member, since a virtual function call relies on a
specific object for determining which function to invoke. A virtual
function can be declared a friend in another class. A virtual func
tion declared in a class shall be defined or declared pure
(_class.abstract_) in that class.
7 Following are some examples of virtual functions used with multiple
base classes:
struct A {
virtual void f();
};
struct B1 : A { // note non-virtual derivation
void f();
};
struct B2 : A {
void f();
};
struct D : B1, B2 { // D has two separate A sub-objects
};
void foo()
{
D d;
// A* ap = &d; // would be ill-formed: ambiguous
B1* b1p = &d;
A* ap = b1p;
D* dp = &d;
ap->f(); // calls D::B1::f
dp->f(); // ill-formed: ambiguous
}
In class D above there are two occurrences of class A and hence two
occurrences of the virtual member function A::f. The final overrider
of B1::A::f is B1::f and the final overrider of B2::A::f is B2::f.
8 The following example shows a function that does not have a unique
final overrider:
struct A {
virtual void f();
};
struct VB1 : virtual A { // note virtual derivation
void f();
};
struct VB2 : virtual A {
void f();
};
struct Error : VB1, VB2 { // ill-formed
};
struct Okay : VB1, VB2 {
void f();
};
Both VB1::f and VB2::f override A::f but there is no overrider of both
of them in class Error. This example is therefore ill-formed. Class
Okay is well formed, however, because Okay::f is a final overrider.
9 The following example uses the well-formed classes from above.
struct VB1a : virtual A { // does not declare f
};
struct Da : VB1a, VB2 {
};
void foe()
{
VB1a* vb1ap = new Da;
vb1ap->f(); // calls VB2:f
}
10Explicit qualification with the scope operator (_expr.prim_) sup
presses the virtual call mechanism. For example,
class B { public: virtual void f(); };
class D : public B { public: void f(); };
void D::f() { /* ... */ B::f(); }
Here, the function call in D::f really does call B::f and not D::f.
10.4 Abstract classes [class.abstract]
1 The abstract class mechanism supports the notion of a general concept,
such as a shape, of which only more concrete variants, such as circle
and square, can actually be used. An abstract class can also be used
to define an interface for which derived classes provide a variety of
implementations.
2 An abstract class is a class that can be used only as a base class of
some other class; no objects of an abstract class can be created
except as sub-objects of a class derived from it. A class is abstract
if it has at least one pure virtual function (which might be inher
ited: see below). A virtual function is specified pure by using a
pure-specifier (_class.mem_) in the function declaration in the class
declaration. A pure virtual function need be defined only if explic
itly called with the qualified-id syntax (_expr.prim_). For example,
class point { /* ... */ };
class shape { // abstract class
point center;
// ...
public:
point where() { return center; }
void move(point p) { center=p; draw(); }
virtual void rotate(int) = 0; // pure virtual
virtual void draw() = 0; // pure virtual
// ...
};
An abstract class shall not be used as an parameter type, as a func
tion return type, or as the type of an explicit conversion. Pointers
and references to an abstract class can be declared. For example,
shape x; // error: object of abstract class
shape* p; // ok
shape f(); // error
void g(shape); // error
shape& h(shape&); // ok
3 Pure virtual functions are inherited as pure virtual functions. For
example,
class ab_circle : public shape {
int radius;
public:
void rotate(int) {}
// ab_circle::draw() is a pure virtual
};
Since shape::draw() is a pure virtual function ab_circle::draw() is a
pure virtual by default. The alternative declaration,
class circle : public shape {
int radius;
public:
void rotate(int) {}
void draw(); // a definition is required somewhere
};
would make class circle nonabstract and a definition of circle::draw()
must be provided.
4 An abstract class can be derived from a class that is not abstract,
and a pure virtual function may override a virtual function which is
not pure.
5 Member functions can be called from a constructor of an abstract
class; the effect of calling a pure virtual function directly or indi
rectly for the object being created from such a constructor is unde
fined.